B-cell lymphoma accounts for 88% Non-Hodgkin's lymphoma (NHL) which is the most common hematological malignancy in adults. The standard therapy for B-cell lymphoma is the combination of rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone (i.e. R-CHOP). However, 60% of responsive patients develop resistance to rituximab and chemotherapy. Acquired drug resistance, therefore, is one of the major obstacles to successfully treating patients with NHL. Results of the prospective multicenter phase III Collaborative trial in relapsed aggressive lymphoma (CORAL) stressed the need to for novel therapeutic strategies to treat relapsed/refractory lymphoma. The p53 pathway plays a pivotal role in drug response: wild type p53 patients respond to R-CHOP well while p53-mutated patients respond poorly. Among the p53- mutated patients, Mdm2 overexpression predicts inferior prognosis. Therefore, targeting Mdm2 E3 ligase and induction of cell death in p53-mutated patients are attractive therapeutic strategies for improved outcome in relapsed/refractory patients. In this regards, Inhibitors targeting the E3 ligase activity of Mdm2-MdmX E3 complex would elicit p53-dependent and p53-independent apoptosis in lymphomas because (1) Mdm2-MdmX E3 complex plays an essential role in negative regulation of p53 and their inhibition will activate p53 and (2) Mdm2-MdmX are involved in p53-independent anti-death mechanisms. Therefore, our central hypothesis is that targeting Mdm2-MdmX E3 ligase activity is a new strategy to treat relapsed/refractory lymphoma via p53- dependent and p53-independent mechanisms. The inhibitors for Mdm2-MdmX E3 ligase will provide new options for lymphoma patients who are resistant to R-CHOP therapy in clinics. However, there are no inhibitors available for Mdm2-MdmX E3 complex. In a high throughput screening effort, we successfully identified small molecule inhibitors that specifically inhibit Mdm2-MdmX E3 activity (designated as MMRi) and they induce p53-dependent and p53-indpendent apoptosis in drug-resistant lymphoma cells. The objective of this application is to examine the therapeutic efficacy of MMRi36 in xenograft lymphoma models of rituximab- resistant lymphoma and in transgenic models of MdmX-driven lymphoma in Aim1 and elucidate the mechanisms of action of MMRi in Aim2 and resolve the 3-D structures of Mdm2-MdmX-MMRi complex in Aim3 for lead optimization and clinical translation of MMRi. The long term goal of this research is to develop targeted therapies and their combinations for better treatment of drug-resistant lymphoma. The proposed research is highly innovative because our MMRis are first-in-class inhibitors with potent anti-lymphoma activity, particularly against drug-resistant lymphoma cells. The proposed research is significant because it addresses a critical gap in lymphoma management and may lead to development of a novel targeted therapy for recurrent lymphoma and provide a new option for ~60% relapsed lymphoma patients after R-CHOP.